A major goal of any X-ray radiographic examination is to record, on the film, perceptible differences in X-ray absorption in a nonhomogenous specimen. The specimens of interest herein are structural and industrial materials that are to be inspected for internal defects, flaws structural faults and the like. A specimen to be tested is positioned between a source of X-ray radiation and a radiographic film. Radiation passed through such a specimen is incident on the emulsions of the film, and the amount of such incident radiation determines the degree of blackening or density of the exposed film. Differences in X-ray absorption by the specimen are accentuated on the film by controlling the total amount of radiation impinging thereon so that a certain film density is attained. The desired film density is the density at which the greatest change occurs for a change in the relative exposure. This desired value can be found by inspecting the H-D curve (plotting the density verses a log function of relative exposure), for the X-ray film and choosing a density where the slope of the curve is the greatest. For most commercially available industrial X-ray films, the maximum slope or region of maximum film sensitivity, occurs between density values of about 1.5 to about 3.5.
Absorption of X-rays by a specimen, of course, varies greatly between specimens of different material types (atomic structure) and of different material thicknesses. To achieve an image on the X-ray film, which image has sufficient film contrast and clarity to denote flaws, a radiographer usually goes through the following standard procedure. First, based on his experience with a particular X-ray machine and the type and thickness of the specimen to be examined, the radiographer chooses the kilovoltage and milliamperage setting on the X-ray machine, the film-to-source distance, and the exposure time. Different X-ray film types and different film intensifying screens can be used if desired. An exposure is then made with the specimen in place and the X-ray film is developed using known film processing methods. If the resulting film density is not within the maximum slope portion of the H-D curve, which happens frequently, one of the above-mentioned variables, typically the kilovoltage setting of the X-ray machine, is adjusted and another exposure is made. This step is repeated until a usable X-ray density value is achieved. Once the resulting X-ray film density falls within the useful portion of the H-D curve for the particular film used, the radiographer then is able to correct or enhance the film image by adjusting one of the above mentioned variables following known procedures.
When the radiographer is satisfied with the film contrast and clarity, he records for his future use the following information: (a) the specimen thickness and material type (its physical density and perhaps the atomic nature of its composition); (b) kilovoltage and milliamperage settings on the X-ray machine; (d) exposure time; (e) the X-ray source-to-film distance, (f) the film type; and (g) the X-ray machine used. Unfortunately, this information cannot be catalogued and used for different X-ray machines because the design and construction of individual X-ray machines are so widely different that they frequently produce X-ray beams of different intensity and spectral content, even when operated at the same stated values of kilovoltage and milliamperage. Thus, it is necessary to treat each X-ray machine on an individual basis.
These procedures are extremely time-consuming, waste a considerable amount of expensive X-ray film, and require elaborate records and record-keeping procedures to ensure future efficient use of the X-ray machine with similar specimens. The availability of extensive records and the radiographer's skill and experience to a large extent determine whether X-ray radiography is a cost effective method for flaw detection of structural and industrial specimens.
Recent developments in the industrial X-ray field have attempted to overcome the foregoing disadvantages. One suggested approach has been to use a suitably positioned ionization chamber to measure the amount of radiation impinging upon and passing through the X-ray film. The radiation intensity impinging upon the X-ray film, as measured by the ionization chamber, is quantified and accumulated. When the accumulated dose of radiation reaches a predetermined value, the X-ray machine is shut off. See Westerkowsky U.S. Pat. No. 3,792,267, entitled Automatic X-Ray Exposure Device. In the Westerkowsky patent, the predetermined value of accumulated dosage for desired film density is selected from a graph of density versus exposure dose to the log 10, for a particular film-type and film foil combination, and for a selected kilovoltage setting on the X-ray machine. Yet, it is unclear from Westerkowsky how the density on the X-ray film varies with respect to kilovoltage. Moreover, the accumulation of detected radiation impinging upon the ionization chamber does not assure the radiographer that an adequate exposure of the specimen will be achieved. The best contrast in the X-ray film is achieved by using the lowest practical kilovoltage setting on the X-ray machine. In Westerkowsky the kilovoltage setting may be entirely too high and the resulting exposure time entirely too short to produce adequate exposure of the specimen with sufficient film contrast to enable detection of flaws within the specimen. Another problem with simply accumulating the radiation is that a selected kilovoltage setting may yield an adequate exposure of the specimen, but the resulting exposure time may be too long to be practical. That is, such prior art X-ray exposure systems do not permit a balancing of a low kilovoltage setting to enhance the exposure of the specimen with a practical exposure time so that the system is cost effective.
It is therefore an object of this invention to provide a new and improved radiographic material inspection apparatus and method that eliminates the need for time-consuming and costly trial exposures.
It is another object of this invention to provide such radiographic apparatus and method that can be used to quickly determine the optimum X-ray tube voltage setting and a correlative practical exposure time.